1. Field of the Disclosure
This disclosure relates generally to drilling of wells into earth formations, and more particularly to the determination of formation properties in situations where the earth formations are anisotropic.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. The drill string may be a jointed rotatable pipe or a coiled tube. Boreholes may be drilled vertically, but directional drilling systems are often used for drilling boreholes deviated from vertical and/or horizontal boreholes to increase the hydrocarbon production. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, tool azimuth, tool inclination. Also used are measuring devices such as a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as measurement-while-drilling (MWD) or logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine formation geology and formation fluid conditions during the drilling operations.
Boreholes are usually drilled along predetermined paths and proceed through various formations. A drilling operator typically controls the surface-controlled drilling parameters during drilling operations. These parameters include weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed (r.p.m. of the surface motor coupled to the drill pipe) and the density and viscosity of the drilling fluid. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to properly control the drilling operations. For drilling a borehole in a virgin region, the operator typically relies on seismic survey plots, which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator may also have information about the previously drilled boreholes in the same formation.
In order to maximize the amount of recovered oil, boreholes are commonly drilled in a substantially horizontal orientation in close proximity to the oil water contact, but still within the oil zone. U.S. Pat. No. RE35386 to Wu et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches a method for detecting and sensing boundaries in a formation during directional drilling so that the drilling operation can be adjusted to maintain the drillstring within a selected stratum is presented. The method comprises the initial drilling of an offset well from which resistivity of the formation with depth is determined. This resistivity information is then modeled to provide a modeled log indicative of the response of a resistivity tool within a selected stratum in a substantially horizontal direction. A directional (e.g., horizontal) well is thereafter drilled wherein resistivity is logged in real time and compared to that of the modeled horizontal resistivity to determine the location of the drill string and thereby the borehole in the substantially horizontal stratum. From this, the direction of drilling can be corrected or adjusted so that the borehole is maintained within the desired stratum. The resistivity sensor typically comprises a transmitter and a plurality of sensors. Measurements may be made with propagation sensors that operate in the 400 kHz and higher frequency.
The hardware used by Wu is a multiple propagation resistivity (MPR) device, schematically illustrated by the example in FIG. 2A. An exemplary tool may include an electronics module 200, two receiver coils 213 and 215, and two pairs of transmitter coils 209, 211 and 217, 219. Such a device has axially oriented coils and has no azimuthal sensitivity. U.S. Pat. No. 6,092,024 to Wu, having the same assignee as the present disclosure, showed that by making redundant measurements with such a device, it was possible to determine the properties of anisotropic media without ambiguity by using complicated processing.
U.S. patent application Ser. No. 11/298,255 of Yu et al., having the same assignee as the present disclosure, teaches the use of a resistivity logging tool having azimuthal sensitivity and illustrated in FIG. 2B. The tool comprises two transmitter coils 251, 251′ whose dipole moments are parallel to the tool axis direction and two receiver coils 253, 253′ that have dipole moments perpendicular to the transmitter direction. In one embodiment of the disclosure, the tool operates at 400 kHz frequency. When the first transmitter is activated, the two receivers measure the magnetic field produced by the induced current in the formation. This is repeated for, the second transmitter. The signals are combined in following way:HT1=H2−(d1/(d1+d2)3·H1 HT2=H1−(d1/(d1+d2))3·H2  (1).Here, H1 and H2 are the measurements from the first and second receivers, respectively, and the distances d1 and d2 are as indicated in FIG. 2B. The tool rotates with the BHA and in an exemplary mode of operation, makes measurements at 16 angular orientations 22.5° apart. The measurement point is at the center of two receivers. In a uniform, isotropic formation, no signal would be detected at either of the two receivers. The device thus makes use of cross component measurements, called principal cross-components, obtained from a pair of transmitters disposed on either side of at least one receiver. It should further be noted that using well known rotation of coordinates, the method of the present disclosure also works with various combinations of measurements as long as they (i) correspond to signals generated from opposite sides of a receiver, and, (ii) can be rotated to give the principal cross components. This device and its variants are referred to as an azimuthal propagation resistivity (APR) tool.
U.S. patent application Ser. No. 11/489,875 of Wang et al., having the same assignee as the present disclosure, disclose a tool which has the conventional propagation resistivity tool together with the azimuthal propagation resistivity tool of Yu. With such a combination, it is possible to obtain a pseudo-image of the earth formation. Additionally, it should be noted that the emphasis in Yu and in Wang is on reservoir navigation and determining a distance to an interface in the earth formation and little effort is spent on the determination of the anisotropic resistivity properties of the earth formation. The present disclosure addresses this need.